1. Technical Field
The present invention relates to a diaphragm, a spherical-shell diaphragm and an electroacoustic transducer, and a method of manufacturing an electroacoustic transducer.
2. Description of the Related Art
An electroacoustic transducer, which is provided with a diaphragm, a frame supporting the periphery of the diaphragm, and a driving section with a magnetic circuit which vibrates the diaphragm in a uniaxial direction, and emits a sound, has been generally called a speaker. Such a diaphragm has had quite often a shape of a cone.
Generally, the surface profile of this cone shape is a nearly circular truncated cone, wherein the cone is obtained by cutting off the top portion of a nearly circular cone (right circular cone) with rotational symmetry about a central axis, and has inner and outer peripheries and an inclined portion inclined to the central axis which is in the vibrating direction.
Incidentally, it has been known that, in the case of the diaphragm with a shape of the nearly circular truncated cone, standing waves are generated in the radius direction all around the diaphragm, and there are easily caused a peak and a dip in frequency—sound pressure characteristics.
Then, in order to hinder such a peak and dip from being caused, there has been proposed a circular truncated cone that is configured in a way that the central axis of the inner periphery is eccentric from that of the outer one.
In the truncated circular cone having such eccentricity, a distance between the inner periphery end and the outer periphery end, the distance being measured along a line passing through the center axis of the inner periphery, is not constant along their circumferential direction.
Accordingly, the wavelength of a standing wave generated in the diaphragm is changed according to the circumferential positions, and a peak and a dip in the frequency—sound pressure characteristics are smoothened out.
As described above, the diaphragm with eccentricity may control the generation of a peak and a dip in the frequency—sound pressure characteristics. However, it is somewhat disadvantageous in that a general-purpose frame or a component for a magnetic circuit cannot be used for such a diaphragm, because such a frame or a component has an outer shape that is not eccentric with regard to the central axis of the inner periphery.
Thereupon, a structure without the above-described disadvantage has been described in Japanese Patent Application Laid-Open Publication No. H09-284886. According to this structure, the above-described diaphragm with eccentricity is installed in a frame or a component for a magnetic circuit, the outer shape of the component or the frame being not eccentric.
On the other hand, there has been known an electroacoustic transducer, which is provided with a diaphragm with a nearly spherical shape, and one driving section (magnetic circuit section) vibrating the diaphragm in the radial direction, and emits a sound in all directions from the center of the diaphragm.
This electroacoustic transducer is called a spherical speaker, a pulsating sphere speaker, or the like, and, has been disclosed in, for example, Japanese Patent Application Laid-Open Publications Nos. 2000-78686, and 2001-95088.
These publications have described, as one example of the diaphragm of this electroacoustic transducer, a diaphragm obtained by combining a plurality of diaphragms with a predetermined shape of approximately a plane into a shape of a nearly sphere.
Incidentally, Japanese Patent Application Laid-Open Publication No. H09-284886 has disclosed that a speaker having an edge member to be connected with the outer periphery of the diaphragm, the edge member having different widths in the radial direction, and a speaker having an edge member and a diaphragm installing member, wherein the edge member has a constant width in the radial direction whereas the diaphragm installing member connecting the above edge member and a frame has different widths in the radial direction, in order to install the diaphragm of which outer shape is eccentric with regard to the center in a frame in which a magnetic circuit is fixed at the center.
However, the above-described configurations have had a problem that, since neither the edge portion nor the diaphragm installing member are a diaphragm, and contribute to radiation of a voice, the projected area of the diaphragm becomes small to the size of the outer shape of the frame, and it is difficult, considering the size of the frame, to obtain a high sound-pressure in a range of low-pitched sound, and also to perform electroacoustic conversion of an input signal with a high efficiency.
Moreover, there has been a problem that, since the outer shape of the diaphragm is eccentric to the center of the magnetic circuit, the radiating axis of a voice does not intersect perpendicularly to the installed surface of the speaker, and it is difficult to specify a reference direction for a radiated voice and also to have a uniform directivity in the circumferential direction.